The present invention relates to a current holding circuit of a two-wire instrument to which a communication device is connected.
FIG. 3 is a circuit diagram showing a two-wire instrument comprising a transmitter (Tx) 1 located at a measurement site 2, and a power source 4 and a receiving resistor R as a receiver located in an instrument room 3 separated away from the measurement site 2. For example, the transmitter 1 detects a pressure difference such as a pressure change in a pipeline as a process variable and transmits detection data. The transmitter 1 transmits an analog current signal of 4 to 20 mA to the remote instrument room 3. This signal is received as a loop current change by the receiving resistor R. That is, the signal from the transmitter 1 is received by a voltage across the receiving resistor R.
A communication device 5 is generally of a portable type and is connected between two-wire loops (loop lines L1 - L2) at the site 2 as shown in FIG. 3, thereby communicating with the transmitter 1. In this case, communication is mainly a maintenance matter such as a parameter change or adjustment in the transmitter 1. Such a communication device is disclosed in Japanese Patent Laid-Open No. 58 (1983)-85649 (corresponding to USSN 317,083 filed on Nov. 2, 1981 entitled "Data Communication Apparatus and Method of Alternately Communicating Digital and Analog Data").
That is, a current is digitally flowed through the communication device 5 to change the voltage across the receiving resistor R, and this voltage change is detected by the transmitter 1 to perform communication. For this reason, while the communication device 5 is operated any current variation appears in the receiving resistor R as a received signal. Therefore, a loop current between the two-wire loops cannot be correctly held.